Continuous fiber granulate and procedure and device for manufacturing a long-fiber granulate

ABSTRACT

Continuous strand pellets made of particles, in which reinforcing staple fibers are arranged in a thermoplastic matrix in a helical manner, are provided. The reinforcing staple fibers and the molten matrix material are situated in a peripheral area of the particles and, together with unmolten thermoplastic staple fibers, are present in a core area of the particles. A method for producing the continuous strand pellets from a staple fiber mixture of thermoplastic fibers and reinforcing fibers includes feeding the staple fiber mixture through a preheating zone and then drawing the staple fiber mixture through a heating nozzle to form a strand, which after rotating and consolidation by cooling, is cut into continuous strand pellets.

[0001] The invention relates to a continuous fiber granulate consistingof granulate corns in which reinforcing staple fibers are helicallyarranged in a thermoplastic matrix. The invention also relates to aprocedure for manufacturing continuous fiber granulate out of a staplefiber mixture of thermoplastic fibers and reinforcing fibers, in which afiber strip comprised of the staple fiber mixture is guided through apreheating zone, then extruded through a heating nozzle into a strand,and the strand is cut into continuous fiber granulate sections afterrotation and consolidation via cooling. Finally, the invention relatesto a device for manufacturing continuous fiber granulate out of a staplefiber mixture of thermoplastic fibers and reinforcing fibers with apreheating zone, a heating nozzle, a cooling zone, a rotating elementand a granulator.

[0002] Chopped fiber-containing granulates are manufactured bycompounding reinforcing fibers and the respective matrix material usingan extruder. The reinforcing fibers can be metered into the extrudercontinuously or chopped. The shearing action of the endless screwsshortens the reinforcing fibers. At the same time, the reinforcingfibers are finely distributed in the polymer melt and cross-linked withthe matrix material. The reinforcing fibers have no aligned orientationin the granulate corn. In these chopped-fiber granulates, the fiberlength of the reinforcing fibers is generally smaller than the granulatelength, normally lying under one millimeter.

[0003] Continuous fiber granulates are increasingly being used in theplastics-processing industry for the manufacture of slightly stiff fibercomposite components. These continuous fiber granulates can basically bemanufactured in different ways. One long-established method is thepultrusion procedure. This procedure is based on the continuous feedingof reinforcing fiber strands (rovings) into a tool while simultaneouslysupplying melted matrix material. In ideal cases, the polymer meltpenetrates through the rovings, and the individual filaments aresheathed with a melt film. At the tool outlet, the strand is passedthrough a nozzle, giving rise to a defined cross-section. After this,the matrix material is cooled, and the material strand is cut to length.The fiber length in a granulate fabricated in this way corresponds tothe granulate length due to the stretched position of the fibers.

[0004] The melt pultrusion procedure is known in numerous variations,and is used both for manufacturing continuous fiber granulate andfabricating semi-finished products (JP-PS 08047924, JP-PS 06320536, U.S.Pat. No. 3,993,726, GB-PS 1,439,327, JP-PS 06315931). Also known arepultrusion procedures for prepreg processing. In these specialpultrusion procedures, melted polymer material need not be supplied,provided that pre-impregnated reinforcing fiber rovings (prepregs) areused.

[0005] Also known is the manufacture of continuous fiber granulatesusing the extrusion procedure (JP-PS 6-254 847 A). In this case, akinking or entanglement of reinforcing fibers can make the length ofindividual fibers greater than the granulate length. However, most ofthe reinforcing fibers are shorter than the granulate length owing tothe shearing action of the extruder screws.

[0006] However, specific reinforcing fiber compositions (rovings orfree-flowing continuous fibers) are always necessary for manufacturingcontinuous fiber granulate in a pultrusion or extrusion process. Thiscannot be accomplished with all reinforcing fibers.

[0007] In addition to the long-known procedures for continuous fibergranulate manufacture, another procedure was also developed for themanufacture of continuous fiber pellets out of fiber strips (DE-PS 19711 247). This procedure is based on the principle of forming a heated,rotated material strand out of a staple fiber strip, which can then becooled and cut into pellet sections. In this procedure, the strand isrotated with two rotating elements that rotate at the same speed. Thedisadvantage to this procedure is that strand rotation is only constantbetween the two rotating elements. After the strand passes through thesecond rotating element and is clamped in the granulator, it mightrotate in the reverse direction given an inadequate cooling and fixationof the material. This has a negative influence on the strength andfree-flowing properties of the granulate.

[0008] The object of this invention is to provide a continuous fibergranulate that has good free-flowing properties, enables a uniformdistribution of reinforcing fibers, and incorporates as large areinforcing fiber length as possible at the given cut length of thegranulate corn. In addition, a procedure for manufacturing continuousfiber granulate out of a staple fiber mixture of thermoplastic fibersand reinforcing fibers is to be provided in which the expended energy islowered. In particular, this procedure is intended to avoid the reverserotation by the strand after passing the cooling zone. Additionaladvantages of the invention are mentioned in the description below.

[0009] This object is achieved according to the invention for thecontinuous fiber granulate mentioned at the outset by having thereinforcing staple fibers be located in a sheathed zone of the particlesalong with the melted matrix material, and in a core zone of theparticles along with unmelted thermoplastic staple fibers. The mainadvantage to this granulate is that reinforcing fibers can beincorporated into the granulate corn with a greater fiber length thanthe granulate section length. It was surprisingly shown that reinforcingfibers with a fiber length greater than the granulate corn sectionlength due to torsion can be incorporated into the particle even if thestrand or granulate corn is not melted through in the core area. Inaddition, it was shown that, despite the only partial melting of thethermoplastic portion (in the sheath or edge zone), no separation takesplace between the reinforcing fibers and matrix material, i.e., thereinforcing fiber load remains essentially the same in the sheath andcore. This makes it possible to achieve a very uniform distribution ofthe reinforcing fibers in the mold units during later processing of thegranulate. The energy expended to manufacture the continuous fibergranulates according to the invention is less than during extrusion orpultrusion (the manufacture of conventional granulates), since thethermoplastic polymer material need not be completely melted, but ratheronly the outside sheath of the strand or granulate corn.

[0010] The granulate particles are preferably strand sections, and thelength of their reinforcing fibers is greater than the length of thestrand sections. The ratio between the lengths of the reinforcing fibersand strand sections depends on the torsion of the strand, and rises astwisting increases. The diameter of the granulate particles generallyranges from 1 to 10 mm. The percentage of reinforcing fibers in thegranulate particles can lie in a range of 10 to 80%w/w. The number ofreinforcing fiber windings in the granulate particles can vary withinbroad limits, and generally lies within a range of 0.1 to 5.

[0011] The reinforcing staple fibers can be natural fibers, syntheticfibers or mineral fibers. Suitable fibers include flax, hemp and jutefibers, along with glass, aramide and carbon fibers. In particular, thematerial of the matrix and thermoplastic fibers can consist ofpolypropylene, polyethylene and polyamide.

[0012] The object is additionally achieved according to the invention inthe procedure mentioned at the outset by first cooling the strandexiting the heating nozzle, then rotating and withdrawing it, andsubsequently allowing it to rotate freely as it is cut into sectionswith no impediment. As opposed to the procedure described in DE 197 11247, the strand is twisted in conjunction with the withdrawal aftercooling is complete, i.e., consolidation. Strand twisting extendsthrough the cooling zone into the heating nozzle, and is limited byfriction against the nozzle all. The design of the granulator ensuresthat the strand can freely rotate even during the cutting process. Areverse rotation of the strand in the area between the rotating elementand granulator is avoided, thereby avoiding a reduction in thefree-flowing properties and strength of the granulate particles.

[0013] In the preferred embodiment of the procedure, the thermoplasticfiber portion of the fiber strip is melted in the preheating zone onlyin a sheath area. This not only saves on heating energy, but alsoaccelerates the procedure, because warming and recooling take less time.Even though the core is not melted, the fibers are sufficientlystabilized in a twisted form therein.

[0014] It is best to use a ratio of speed to withdrawal rate rangingfrom 10 to 150 revolutions per meter for the strand. In particular, thisratio lies between 20 and 60 revolutions per meter.

[0015] The reinforcing fiber percentage in the fiber strip preferablylies within the range of 10 to 80%w/w, in particular between 15 and50%w/w. An average reinforcing fiber length in the fiber strip rangingfrom 30 to 200 mm is best selected. The fineness of the fiber stripgenerally ranges from 5 to 30 ktex. The strand is preferably cut to alength ranging from 1 to 30 mm, with this length in particular lyingbetween 10 and 30 mm.

[0016] Finally, the object is achieved in the device defined at theoutset by having the rotating element that generates the strand torsionbe arrange behind the cooling zone and also be designed as thewithdrawal element for the strand, and by having the granulator exert notorque, or at most a torque without permanent reverse rotation, on thestrand during the separation process. The granulator is preferably setup to execute essentially torque-free cutting operations. Avoiding thereverse rotation yields a granulate characterized by strength andfree-flowing properties, even though the core has not melted through.The granulator can operate in such a way that the elements envelopingand cutting the strand together rotate at roughly the same speed as therotating element.

[0017] The preheating zone is best set up only for melting the sheathzone of the staple fiber strip. In like manner, the cooling zone is setup only for cooling and consolidating the melted sheath zone.

[0018] In the following, the invention will be described in greaterdetail based on the drawing and embodiments. Shown on:

[0019]FIG. 1 is a diagrammatic side view of a granulate particle in theform of a cylindrical strand section, wherein the reinforcing fibers areonly partially penciled in;

[0020]FIG. 2 is the cross section of the granulate particle shown onFIG. 1, and

[0021]FIG. 3 is a diagrammatic view of the device according to theinvention for manufacturing the continuous fiber granulate.

[0022] As evident from FIG. 1, the reinforcing fibers 11 are arranged ina helical thread in the section, wherein the fibers 11 are embedded inthe melted matrix material 12 in the sheath zone. FIG. 2 shows a crosssection of the core 13 comprised of reinforcing fibers and thermoplasticmatrix fibers, and a sheath 14 consisting of reinforcing fibers 11 in amelted matrix material 12.

[0023] In FIG. 3, the device comprises a storage tank 1, from which thefiber strip 2 consisting of thermoplastic matrix fibers and reinforcingfibers is fed to a preheating zone 4 via withdrawal rolls 3. In thepreheating zone 4, the matrix fibers are melted within a sheath zone ofthe strip. In this case, less energy is required as during completemelting, and the strand rate can be accelerated. After leaving thepreheating zone 4, the heated material strand passes through a heatednozzle 5, which effects a calibration of the strand cross section. Thestrand then passes through a cooling zone 6 to further cool andconsolidate the matrix material. The strand is rotated at the end of thecooling zone 6 by a combined withdrawal and rotating element 7. The corepiece of the combined withdrawal and rotating element 7 is comprised ofrolls that rotate against each other to withdraw the strand locatedbetween them and rotate around the strand axis for twisting purposes.The design of such a combined twisting unit is known from the textileindustry, and has been described in the literature (W. Wegener, “DieStreckwerke der Spinnereimaschinen” (Drawing Equipment in SpinningFrames).

[0024] The rotation of the material strand extends from the rotatingelement 7 through the cooling zone 6 up to the heating nozzle 5, and islimited by the friction against the nozzle wall. The strand is cut intogranulate particle (continuous fiber granulate) sections using agranulator 8. In this case, the design of the granulator ensures thatthe strand can also rotate freely during the cutting process. Thegenerated granulate particles have a core-sheath structure (consolidatedsheath/fiber material in core). The strand can be twisted in a definedmanner in a continuous procedure using this device.

[0025] Embodiment 1

[0026] A fiber strip consisting of flax and polypropylene fibers with aband fineness of 10 ktex, an average fiber length of the flax andpolypropylene fibers of 60 mm and a flax fiber content of 30%w/w isheated to a temperature of 250° C. in the preheating passage of thesystem shown on FIG. 3. After leaving the preheating passage, the fiberstrip passes through a nozzle heated to 250° C. with a diameter of 5 mm.After exiting the nozzle, the fiber strand passes through a downstreamcooling passage, and is torqued at 50 revolutions per meter at the endof the cooling zone by a combined withdrawal and rotating element.Finally, the material strand is cut into pellets with a length of 20 mmusing a granulator.

[0027] Embodiment 2

[0028] A fiber strip consisting of hemp and polypropylene fibers with aband fineness of 8 ktex, an average fiber length of 40 mm and a hempfiber content of 20%w/w is heated to a temperature of 230° C. in apreheating passage by means of hot air. After leaving the preheatingpassage, the fiber strip passes through a nozzle heated to 230° C. witha diameter of 4 mm. After exiting the nozzle, the fiber strand passesthrough a cooling passage for consolidating the melted sheath zone, andis torqued at 30 revolutions per meter by a combined withdrawal androtating element. The strand is then cut into pellets with a length of10 mm using a granulator.

[0029] Embodiment 3

[0030] A continuous fiber granulate consisting of a fiber mixture ofglass reinforcing fibers and polyamide matrix fibers according to theinvention with a glass fiber content of 40 %w/w, a granulate diameter of3 mm, a cut length of 10 mm, and a rotation of 50 revolutions per meteris processed on an injection molding machine. The homogeneousdistribution of the reinforcing fibers and helical fiber arrangementmakes it possible to meter in large reinforcing fiber lengths andmanufacture mold units with a uniform reinforcing fiber distribution inthe polyamide mass.

[0031] Embodiment 4

[0032] A continuous fiber granulate consisting of a fiber mixture offlax reinforcing fibers and polypropylene matrix fibers with a flaxfiber content of 30%w/w, a granulate diameter of 6 mm, a cut length of25 mm, and a rotation of 40 revolutions per meter is melted on a screwplasticising unit. A material is subsequently placed in a press with theplasticising unit and pressed into a large-surface mold part. Thehelical fiber arrangement makes it possible to meter in a largereinforcing fiber length with no problem. In addition, the uniformdistribution of the reinforcing fibers makes it possible to achieve auniform fiber content in the entire molding.

1. Procedure for manufacturing continuous fiber granulate out of astaple fiber mixture of thermoplastic fibers and reinforcing fibers, inwhich a fiber strip comprised of the staple fiber mixture is guidedthrough a preheating zone, then extruded through a heating nozzle into astrand, and the strand is cut into continuous fiber granulate sectionsafter rotation and consolidation via cooling, characterized by the factthat the strand is first cooled, and then rotated, withdrawn andsubsequently allowed to rotate freely as it is cut into sections with noimpediment.
 2. Procedure according to claim 1, characterized by the factthat the thermoplastic fiber portion of the fiber strip is only meltedin a sheath area in the preheating zone.
 3. Procedure according to claim1 or 2, characterized by the fact that a ratio of speed to withdrawalrate ranging from 10 to 150 revolutions per meter is used.
 4. Procedureaccording to one of claims 1 to 3, characterized by the fact that thepercentage of reinforcing fibers in the fiber strip ranges from 10 to80%w/w.
 5. Procedure according to one of claims 1 to 4, characterized bythe fact that an average reinforcing fiber length in the fiber stripranging from 30 to 200 mm is selected.
 6. Procedure according to one ofclaims 1 to 5, characterized by the fact that a fineness of the fiberstrip ranging from 5 to 30 ktex is selected.
 7. Continuous fibergranulate consisting of granulate particles, in which reinforcing staplefibers are helically arranged in a thermoplastic matrix, characterizedby the fact that the reinforcing staple fibers are located in a sheathedzone (14) of the particles (15) along with the melted matrix material(12), and in a core zone (13) of the particles along with unmeltedthermoplastic staple fibers.
 8. Continuous fiber granulate according toclaim 7, characterized by the fact that the particles are strandsections (15), and the length of their reinforcing fibers (11) isgreater than the length of the strand sections.
 9. Continuous fibergranulate according to claim 7 or 8, characterized by the fact that thereinforcing staple fibers are natural fibers, synthetic fibers ormineral fibers.
 10. Continuous fiber granulate according to one ofclaims 7 to 9, characterized by the fact that the diameter of thegranulate particles (15) ranges from 1 to 10 mm.
 11. Continuous fibergranulate according to one of claims 7 to 10, characterized by the factthat the percentage of reinforcing fibers (11) in the granulate particle(15) ranges from 10 to 80%w/w.
 12. Continuous fiber granulate accordingto one of claims 7 to 11, characterized by the fact that the material ofthe matrix and thermoplastic fibers preferably consists ofpolypropylene, polyethylene or polyamide.
 13. Device for manufacturingcontinuous fiber granulate out of a staple fiber strip consisting ofthermoplastic fibers and reinforcing fibers, with a preheating zone (4),a heating nozzle (5), a cooling zone (6), a rotating element (7) and agranulator (8), characterized by the fact that the rotating element (7)that generates the strand torsion is arranged behind the cooling zone(6) and designed as a withdrawal element for the strand, and that thegranulator (8) exerts no torque, or at most a torque without permanentreverse rotation, on the strand during the separation process. 14.Device according to claim 13, characterized by the fact that thepreheating zone (4) is set up only for melting a sheath zone of thestaple fiber strip.